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Marine Ecology Progress Series

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MEPS 374:1-6 (2009)  -  DOI:

'Ghost nutrients' from fish farms are transferred up the food web by phytoplankton grazers

Paraskevi Pitta1, Manolis Tsapakis1, Eugenia T. Apostolaki1, Tatiana Tsagaraki1, Marianne Holmer2, Ioannis Karakassis3,*

1Hellenic Centre for Marine Research, Institute of Oceanography, PO Box 2214, 71003 Heraklion, Crete, Greece
2Institute of Biology, SDU-Odense University, Campusvej 55, 5230 Odense M, Denmark
3Marine Ecology Laboratory, Department of Biology, University of Crete, PO Box 2208, 71409 Heraklion, Crete, Greece
*Corresponding author. Email:

ABSTRACT: Several studies have failed to detect significant changes in chl a content of the water column in the vicinity of fish farms despite the large amount of nutrient wastes discharged into the marine environment. To trace the fate of these disappearing (or ‘ghost’) nutrients, experiments using dialysis bags deployed in situ along gradients of nutrient enrichment (at different distances from cage fish farms) were carried out at 2 coastal locations in the eastern Mediterranean. The productivity of the water column decreased with distance from the nutrient discharge point, as indicated by the chl a content found in the dialysis bags. However, comparison of the results from bioassays with and without grazer exclusion showed that grazing plays a key role in regulating phytoplankton biomass, keeping chl a at very low levels and effectively transferring nutrients up the food web. The fact that this type of response was found in otherwise oligotrophic conditions is probably due to the structure of the phytoplankton community in such areas, which is based on small cell-size primary producers that can be easily grazed on by planktonic ciliates.

KEY WORDS: Phytoplankton · Ciliates · Mariculture · Fish farms · Nutrient loading · Dialysis bags · Eutrophication

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Cite this article as: Pitta P, Tsapakis M, Apostolaki ET, Tsagaraki T, Holmer M, Karakassis I (2009) 'Ghost nutrients' from fish farms are transferred up the food web by phytoplankton grazers. Mar Ecol Prog Ser 374:1-6.

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